1. Field of the Invention
This invention relates to the control of synchronous permanent magnet motors, particularly though not exclusively in a downhole environment such as a hydrocarbon (oil, gas, or mixed oil and gas) well.
2. Description of the Related Art
Permanent magnet motors are anticipated to replace the standard induction motor in downhole ESP applications. Due to their power density compared to current technologies and to their high efficiency due their built in excitation, it is expected that they will competing more and more with induction motor based ESP systems.
Brushless permanent magnet motors conventionally comprise a rotor having permanent magnets, and a stator winding which induces the rotor to turn. The motor is supplied with current, which is electronically commutated to energise different parts of the winding as the rotor turns.
Conventional art as exemplified by US 2009/0146592 A1 and JP 2001025282 relies upon closed loop control methods to start and operate a synchronous motor. These involve either direct measurements or estimation of control quantities like position from directly measurable quantities like voltages and currents.
In order to control the rotor, the rotor position is determined. This can be a position sensor to coordinate the variable speed drive device switching with the position of the motor back-EMF. This typically requires a Hall effect device based position sensor, a resolver or encoder to provide three signals shifted 120° that are used by the variable speed drive (VSD) to time the signals to the devices. Due to the harsh nature of the environment, these position sensors are not reliable and may not survive the prevailing well temperature. Alternatively sensorless algorithms have been developed which deduce the rotor position from the motor back emf, which however can be very unreliable at motor start up and at low speeds if the motor is under load.
A synchronous motor runs at the same speed as the supply frequency to its armature. The actual speed in revolution per minute (rpm) is a function of supply frequency and the number of pole pairs of the field. Unlike asynchronous motors, synchronous motors require a start-up sequence to ensure that the rotor gains speed without locking. Direct on-line starting methods seldom work on synchronous machines. The amount of torque to be generated by the interaction of the field with the armature current depends on the magnitude of the rotor field and the stator currents. It is the interaction between these fields that generates the torque required to accelerate the rotor and supply the torque required by the load. Ideally these fields must be in quadrature (90 Electrical Degrees) to maximize the torque generation. So for a successful start, the amount of current supplied to the stator has to be adequate and the rate at which the motor is accelerated needs to be consistent with the torque generated. Conventionally, there are two ways to do this, both being closed-loop methodologies:
(i) A physical position sensor is provided, and maximum torque is ensured by maintaining a 90° angular relationship between the rotor field and armature currents. In ESP applications, the distance of the position sensor, for example, some 3000 m from the drive is prohibitive and furthermore the reliability of such systems is reduced by the addition of additional devices and associated cables.
The position sensor means indicates when the switches and open or closed to supply stator currents, and typically comprises either a Hall effect sensor or a group of optical devices positioned 120° apart. The signals from the three sensors are used to generate the gating signals. In such systems, the current level is controlled to set the torque level so thus acceleration.
(ii) Alternative, sensorless methods make use of direct measurements of voltage and current and from the voltage equation estimate the position by integration methods. Such methods are difficult to use in ESP systems, primarily because of the long cables between the motor and the drive and the fact that in the majority of cases cable impedances are non-symmetrical due to the flat configuration of the cable, which is typically supported by strapping it to the production tubing. The voltages and current are therefore no longer balanced and therefore estimation methods are no longer accurate. Position errors therefore lead to lower stability margin of the system and may lead to the motor losing synch during transients or when there is gas in the well and the motor loses load momentarily.
It is an object of the present invention to provide a more satisfactory way of controlling a synchronous permanent magnet motor, particularly in downhole applications where the conductors are of great length.